This invention relates to devices and methods for detecting an analyte, for example, using an immunoassay.
An analyte in a sample may be detected by treating the sample with various reagents, such as labeled immunological binding partners to the analyte and reagents to enable detection of the label. Often, the sample must be washed between administration of various reagents.
An accurate assay may depend on controlling the amount of reactants exposed to the sample and the duration of the reactions taking place. It is desirable to control these variables in a way that enables use of the assay kit by individuals of widely varying skill and experience, so that variations in individual technique do not materially alter the result. Consistency and accuracy can be difficult, particularly when a color generating indicator must be evaluated against a background. Test reagents should be tested to verify their activity, which may erode over time. Also, it is desirable to reduce the time necessary to perform the assay. It is desirable to have the ability to assay extremely small sample volumes with relative low concentrations of analyte, and/or to detect relatively small differentials in analyte concentration. Finally, it is desirable to assay whole blood samples without complex centrifuging equipment.
One method for adding and washing reagents in an immunoassay uses an absorbent material to move liquid washes and reagents through a solid substrate (such as a membrane) to which other reactants are immobilized.
Cole et al. U.S. Pat. No. 4,246,339, discloses an immunoassay test device including sorbent material for drawing liquid through a microporous membrane at the bottom of a test well. The sorbent material is resiliently biased away from the membrane, and it draws liquid through the membrane only when the two are forced together to overcome the bias. Sorbent material comprises a surface layer which is hydrophobic and a bulk portion which is wettable. Reagents are added serially to the test well and, after each reagent has been in the well for a prescribed time, the membrane and sorbent material are forced together to draw off liquid before the next reagent is added.
Tom U.S. Pat. No. 4,366,241, discloses an immunoassay device having two bibulous zones, an analyte binding partner being non-diffusively fixed in the first zone (the "immunoabsorbing zone") and the second layer being a reservoir zone, either directly or indirectly in liquid-receiving relationship with the first zone to pull liquid through and out of the first zone throughout the duration of the assay. The immunoabsorbing zone is an exposed portion of a flat test strip which is otherwise covered by a protective coating.
Hossom U.S. Pat. No. 4,623,461, discloses a test device having a filter which feeds a specimen to a flat absorbent material having a reaction zone surrounded by peripheral zone. An annular ring of absorbent material is positioned around the peripheral zone, so that fluid is drawn radially outward from the reaction zone, through the peripheral zone, and into the absorbent material.
Bagshawe U.S. Pat. No. 3,888,629, discloses an assay device having a matrix pad which may be pre-treated with a binding partner for the analyte being assayed. Absorbent material is forced into intimate contact with matrix pad to increase the speed of filtration through the matrix pad.
Kondo U.S. Pat. No. 4,270,920, discloses multiple reagent layers arranged on a single horizontal support. A porous spreading layer spreads the sample as it moves into the reagent layers, so that a relatively small sample volume will be spread evenly over the reagent layers.
Lawrence U.S. Pat. No. 4,365,970, Oksman U.S. Pat. No. 4,578,358, Brewer U.S. Pat. No. 4,486,540, and Guandagno U.S. Pat. No. 4,541,987, disclose test devices comprising positive and negative control spots on a test pad or slide.
Applicants' assignee, Idexx Corporation (formerly Agritech Systems, Inc.), has marketed an immunoassay device comprising positive and negative control spots. Separation of the spots is maintained by spotting the reagents a sufficient distance from each other.
Various companies have marketed self-contained immunoassays using a membrane as a solid support (e.g. Syntex's AccuLevel and Hybritech's ICON). See Valkirs U.S. Pat. Nos. 4,632,901 and 4,727,019. See also, Weiss and Blankstein, American Clinical Products Review, May/June (1987). Latex particles, to which ligands have been attached, have been used in aqglutination assays Bangs, American Clinical Products Review, May/June (1987).